Conventionally, a framework composed of a variable gain amplifier (VGA), a sample-and-hold circuit (SHC), and an analog/digital converter (A/D), which are serially connected, is used as a receiving end (Rx) for receiving analog signals. Thus, an analog signal received by the input terminal is first amplified/reduced by the VGA, and then the amplified/reduced signal is sampled by the SHC, and finally the sampled signal is converted by the A/D converted into a digital signal.
There are two kinds of conventional framework of receiving end, that is, the single-line framework and the dual-line framework. Please refer to FIG. 1, which is a schematic diagram depicting a conventional receiving end with single-line framework. As seen in FIG. 1, not only the receiving end 100 is composed of a VGA 110, a SHC 120 and an A/D converter 130, which are serially connected, but also a single-line 115 is used for connecting the VGA 110 and the SHC 120, and a single-line 125 is used for connecting the SHC 120 and the A/D converter 130, since the VGA 110 has only a single output.
Wherein, an analog signal 140 received by the receiving end 100 is first being amplified by the VGA 110, and then the amplified signal is sampled by the SHC 120, and finally the sampled signal is being fed into the A/C converter 130 to be converted into a digital signal.
However, since the analog signal 104 amplified by the VGA 110 carries noise, a receiving end with a dual-line framework is designed to overcome the aforementioned shortcoming.
Please refer to FIG. 2, which is a schematic diagram depicting a conventional receiving end with dual-line framework. As seen in FIG. 2, the receiving end 200 is similarly composed of a VGA 210, a SHC 220 and an A/C converter 230, which are serially connected, but there are dual-lines 213, 215 used for connecting the VGA 210 and the SHC 220, and dual-lines 223, 225 used for connecting the SHC 220 and the A/C converter 230, since the VGA 210 has a pair of differential outputs.
Wherein, an analog signal 140 received by the input terminal framework 200 is first being amplified by the VGA 210, and a pair of amplified differential signals are then sampled by the SHC 220 such that the noise can be eliminated since the amplified differential signals have the noise with the same quantity, and finally the sampled signals are fed into the A/C converter 130 to be converted into a digital signal.
The waveform of the sampled analog signal 140 produced by the SHC of the conventional receiving ends 100 and 200 are shown in FIG. 3. The sine wave analog signal 300 is sampled to form either a square wave signal 310 or a discrete signal 320.
However, if the magnitude of the analog signal 140 amplified by the VGA is too large, the analog signal is likely to oscillate and causes signal distortion since the frequency of the inputted analog signal is the same as that of the feedback analog signal in the feedback loop of the receiving end.
Please refer to FIG. 4, which is a frequency spectrum of a FFT sine wave. As seen in FIG. 4, the two peaks near a specific frequency, i.e. 2500 Hz in a frequency domain represent respectively the wave crest and the wave an exemplified sine wave in a time domain while applying a Fast Fourier Transform on the sine wave analog signal.
Moreover, please refer to FIG. 5, which is a frequency spectrum of a FFT sine wave after it being sampled by the SHC. As seen in FIG. 5, If the sampling frequency of the SHC is far larger than the frequency of the sine wave, the amplitude and the position of two peaks in the frequency domain, which represent respectively the wave crest and the wave of the sampled sine wave in time domain is almost the same with the two peaks shown in FIG. 4.
If the conventional receiving end architecture has a feedback loop, the transfer function of the conventional receiving end can be shown as following:A(f)/(1+A(f)*β(f))                Where A is a gain function;         β is a feedback function; and         f is frequency.Since the conventional receiving end architecture is composed of a VGA, a SHC and an A/D converter, the gain function A(f) can be decomposed into the following:A1(f)*A2(f)*A3(f)        Where A1(f), A2(f), and A3(f) are gain function of the VGA, the SHC and the A/D converter respectively.Those skilled in the art will know that if the gain function of the VGA, that is, the value of A1(f), is too large while the value of f in the equation: A1(f)*A2(f)*A3(f) is the same, the phase margin of the transfer function of the conventional receiving end is as following:1/|(A(f)*β(f))|<1In this manner, the phase margin of the analog signal in the conventional receiving end is smaller than 1 such that the signal is instable and is likely to oscillate causing signal distortion.        
In view of the foregoing description, the present invention provides a receiving end architecture with mix-type sample-and-hold circuit that is capable of reducing the signal distortion caused by the oscillation of analog signal in the receiving end.